1. Field of the Invention
The present invention relates generally to a low loss Diode-ORing circuit, and specifically to a parallel configuration of a diode and a power MOSFET transistor in each diode branch yielding minimum power dissipation and low temperature on the circuit components.
2. Background of the Invention
Redundant system of device power feed is often required when powering MISSION critical equipment. Then two or more power sources must be connected to provide power to the system.
When one of the power sources fails or is removed from the service the other power source is available in a redundant capacity to supply required power to the system without interruption of service. In this situation, it is also important to ensure that disconnected input terminal does not carry potentially dangerous power that can injure service personnel.
Those skilled in the art will be familiar with conventional Diode-ORing circuits providing redundant power to the load. Referring to FIG. 1, a conventional Diode-ORing circuit is commonly used to connect two or more power feed inputs to a single device load. Note that connection of the diodes in FIG. 1 assumes the negative power source as it is commonly used in telecommunication industry. Here and in the rest of this paper the diodes and other devices should be reverse connected if positive power source is used. When one of the power feeds is disconnected, the other feed continues to provide power to the load through its diode. The current would not flow back to the disconnected input due to the diode in the disconnected branch thus isolating that input from the rest of the circuit. The typical Diode-ORing circuit in power application includes two or more diodes each connected in series between an individual power source and common output to the load. Each diode allows current to flow to the load but limits the same current from flowing out of the other feed input. In this manner each input in the circuit is isolated from the other inputs and does not allow generating a potentially dangerous voltage at the disconnected input back fed from the other source(s).
Typically high power Schottky diodes are used in these applications in order to reduce voltage drop across the diodes and, therefore, reduce the power losses. The common problem in this application is still high power dissipation on the Schottky diodes especially when application requires using the diodes with high reverse voltage characteristics. These diodes have relatively higher forward voltage drop characteristics and, therefore, create higher power dissipation. The problem becomes more severe when one input is disconnected and the other should take over to carry full current to the load.
For example, a high power Schottky diode may have forward voltage drop of 0.6 Volts at the load current of 20 Amps each (required load current is 40 Amps) yielding the power dissipation:
P=Vxc3x97I=0.6xc3x9720=12 Watts.xe2x80x83xe2x80x83(EQ.1)
When only one input conducting full load current the voltage drop across the single Schottky diode may increase to 0.9 Volts yielding the power dissipation of:
P=Vxc3x97I=0.9xc3x9740=36 Watts.xe2x80x83xe2x80x83(EQ. 2)
The other problem of conventional circuit in FIG. 1 is concentration of heat on single silicon die. When in the example above both diodes are connected and conducting current to the load, each diode actually conducts only half of the current yielding conductive losses of 12 Watts each. Here each silicon die dissipates only 12 Watts. When one power feed is disconnected the other diode provides full power to the load and dissipates 36 Watts concentrated on one silicon die.
Dissipating such a large amount of power is undesirable because it requires a large heatsink that (1) takes up valuable physical space and (2) undesirably increases the cost of the design. Therefore, there is a need to find an inexpensive Diode-ORing circuit with minimal power losses and heat dissipation.
A simple solution to this problem is merely to select an overrated diode that has yet lower forward voltage drop and higher forward current. This solution reduces the requirements for such a large heatsink, but drive up the cost of the design since the diodes of such characteristics, especially of high reverse voltage, typically very expensive. Given the increased cost of the diodes, it is often unfeasible to competitively implement it in many circuit designs.
Another possible solution is to use a power MOSFET transistor instead of the diode in the Diode-ORing circuit. The equivalent drain-to-source resistance of the transistor, commonly known as Rdson, can be low enough that the power dissipation is improved comparing to the conventional Diode-ORing circuit. In the above example when current is 20 Amps and Rdson is 0.025 Ohms the Power Dissipation is:
P=I2xc3x97R=202xc3x970.025=10 Watts.xe2x80x83xe2x80x83(EQ. 3)
However, as the current increases the power dissipation increases proportionally to the square of the current value. Therefore, while using a transistor instead of the diode at lower current is beneficial, the dissipated power can be much higher when one of the inputs is disconnected and the transistor is forced to provide double the load current:
P=I2xc3x97R=402xc3x970.025=40 Watts.xe2x80x83xe2x80x83(EQ. 4)
Further to exacerbate the problem, the Rdson of the transistor has positive thermal coefficient. It""s value increases with increase of the die temperature that can cause a thermal run-away and eventual destruction of the transistor. To avoid this problem the transistors would have to be placed on even larger heatsink and use of more costly devices would be necessary.
In light of the foregoing, a need exists for a circuit that adequately provides redundant power to the load while minimizing the amount of power losses.
Accordingly this invention is directed to a low loss diode-oring circuit and method of providing redundant power feed to the load with minimal power losses that substantially obviates one or more of the limitations and disadvantages of the described prior arrangements.
Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve these and other objects and advantages, and in accordance with the purpose of the invention as embodied and broadly described herein, a low loss diode-oring circuit consistent with the present invention has:
two or more input feeds (two input feeds are described herein, however, the number of input feeds is not limited in this invention);
a parallel combination of diode, such as a low forward voltage drop diode (such as Schottky diode), and a transistor, such as a low Rdson power MOSFET transistor, in each input branch connected in between each input feed and the output; and
each parallel network of diode transistor combination is placed on the same heatsink, to facilitate heat transfer from transistor to diode and vise versa. Typically, the diode has negative thermal coefficient. That is when the heat dissipated in the transistor indirectly heats the diode. The diode""s forward voltage threshold reduces due to a negative thermal coefficient and the diode begins to conduct earlier in its characteristics. The opposites are true for a MOSFET transistor. Typically it has positive thermal characteristics. Rdson of the transistor would increase with increase of its temperature thus increasing voltage drop across transistor at the same given current and further increasing power dissipation and temperature. This phenomenon may cause a thermal ran-away of the transistor and eventual destruction of the silicon die. The diode connected in parallel with the transistor will begin to conduct xe2x80x9cearlierxe2x80x9d in its characteristics due to the negative thermal coefficient described above. Thus, the diode xe2x80x9ctakes overxe2x80x9d some current from the transistor xe2x80x9crelievingxe2x80x9d it from excessive load current further providing thermally balanced coupling of the diode-transistor network.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.